Methods and Systems for Transitioning an Aerial Vehicle Between Hover Flight and Crosswind Flight

ABSTRACT

A method may involve operating an aerial vehicle in a hover-flight orientation. The aerial vehicle may be connected to a tether that defines a tether sphere having a radius based on a length of the tether, and the tether may be connected to a ground station. The method may involve positioning the aerial vehicle at a first location that is substantially on the tether sphere. The method may involve transitioning the aerial vehicle from the hover-flight orientation to a forward-flight orientation, such that the aerial vehicle moves from the tether sphere. And the method may involve operating the aerial vehicle in the forward-flight orientation to ascend at an angle of ascent to a second location that is substantially on the tether sphere. The first and second locations may be substantially downwind of the ground station.

BACKGROUND

Unless otherwise indicated herein, the materials described in thissection are not prior art to the claims in this application and are notadmitted to be prior art by inclusion in this section.

Power generation systems may convert chemical and/or mechanical energy(e.g., kinetic energy) to electrical energy for various applications,such as utility systems. As one example, a wind energy system mayconvert kinetic wind energy to electrical energy.

SUMMARY

Methods and systems for transitioning an aerial vehicle between certainflight modes that facilitate conversion of kinetic energy to electricalenergy are described herein. Beneficially, embodiments described hereinmay reduce drag on the aerial vehicle, which may reduce thrust of theaerial vehicle for flight. Further, embodiments described herein mayallow a thrust-to-weight ratio of the aerial vehicle to be reduced.Accordingly, the size of one or more components of the aerial vehicle,such as motors or propellers, may be reduced, which may allow the aerialvehicle to be optimized for power generation flight.

In one aspect, a method may involve operating an aerial vehicle in ahover-flight orientation, wherein the aerial vehicle is connected to atether that defines a tether sphere having a radius based on a length ofthe tether, wherein the tether is connected to a ground station; whilethe aerial vehicle is in the hover-flight orientation, positioning theaerial vehicle at first location that is substantially on the tethersphere, wherein the first location is substantially downwind of theground station; transitioning the aerial vehicle from the hover-flightorientation to a forward-flight orientation, such that the aerialvehicle moves from the tether sphere, wherein the aerial vehicle hasattached flow, and wherein a tension of the tether is reduced; andoperating the aerial vehicle in the forward-flight orientation to ascendat an angle of ascent to a second location that is substantially on thetether sphere, wherein the second location is substantially downwind ofthe ground station.

In another aspect, a system may include a tether connected to a groundstation, wherein the tether defines a tether sphere having a radiusbased on a length of the tether; an aerial vehicle connected to thetether; and a control system configured to: operate the aerial vehiclein a hover-flight orientation; while the aerial vehicle is in thehover-flight orientation, position the aerial vehicle at a firstlocation that is substantially on the tether sphere, wherein the firstlocation is substantially downwind of the ground station; transition theaerial vehicle from the hover-flight orientation to a forward-flightorientation, such that the aerial vehicle moves from the tether sphere,wherein the aerial vehicle has attached flow, and wherein a tension ofthe tether is reduced; and operate the aerial vehicle in theforward-flight orientation to ascend at an angle of ascent to a secondlocation that is substantially on the tether sphere, wherein the secondlocation is substantially downwind of the ground station.

In another aspect, a method may involve operating an aerial vehicle in acrosswind-flight orientation, wherein the aerial vehicle is connected toa tether that defines a tether sphere having a radius based on a lengthof the tether, wherein the tether is connected to a ground station;while the aerial vehicle is in the crosswind-flight orientation,positioning the aerial vehicle at a first location that is substantiallyon the tether sphere, wherein the first location is substantiallydownwind of the ground station; transitioning the aerial vehicle fromthe crosswind-flight orientation to a forward-flight orientation, suchthat the aerial vehicle moves from the tether sphere, wherein the aerialvehicle has attached flow, and wherein a tension of the tether isreduced; and operating the aerial vehicle in the forward-flightorientation to ascend at an angle of ascent to a second location,wherein the second location is substantially downwind of the groundstation.

In another aspect, a system may include means for operating an aerialvehicle in a hover-flight orientation, wherein the aerial vehicle isconnected to a tether that defines a tether sphere having a radius basedon a length of the tether, wherein the tether is connected to a groundstation; while the aerial vehicle is in the hover-flight orientation,means for positioning the aerial vehicle at a first location that issubstantially on the tether sphere, wherein the first location issubstantially downwind of the ground station; means for transitioningthe aerial vehicle from the hover-flight orientation to a forward-flightorientation, such that the aerial vehicle moves from the tether sphere,wherein the aerial vehicle has attached flow, and wherein a tension ofthe tether is reduced; and means for operating the aerial vehicle in theforward-flight orientation to ascend at an angle of ascent to a secondlocation that is substantially on the tether sphere, wherein the secondlocation is substantially downwind of the ground station.

In yet another aspect, a system may include means for operating anaerial vehicle in a crosswind-flight orientation, wherein the aerialvehicle is connected to a tether that defines a tether sphere having aradius based on a length of the tether, wherein the tether is connectedto a ground station; while the aerial vehicle is in the crosswind-flightorientation, means for positioning the aerial vehicle at a firstlocation that is substantially on the tether sphere, wherein the firstlocation is substantially downwind of the ground station; means fortransitioning the aerial vehicle from the crosswind-flight orientationto a forward-flight orientation, such that the aerial vehicle moves fromthe tether sphere, wherein the aerial vehicle has attached flow, andwherein a tension of the tether is reduced; and means for operating theaerial vehicle in the forward-flight orientation to ascend at an angleof ascent to a second location, wherein the second location issubstantially downwind of the ground station.

These as well as other aspects, advantages, and alternatives, willbecome apparent to those of ordinary skill in the art by reading thefollowing detailed description, with reference where appropriate to theaccompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 depicts an Airborne Wind Turbine (AWT), according to an exampleembodiment.

FIG. 2 is a simplified block diagram illustrating components of an AWT,according to an example embodiment.

FIGS. 3 a and 3 b depict an example of an aerial vehicle transitioningfrom hover flight to crosswind flight, according to an exampleembodiment.

FIGS. 4 a-c are graphical representations involving an angle of ascent,according to an example embodiment.

FIGS. 5 a and 5 b depict a tether sphere, according to an exampleembodiment.

FIGS. 6 a-c depict an example of an aerial vehicle transitioning fromcrosswind flight to hover flight, according to an example embodiment.

FIG. 7 is a flowchart of a method, according to an example embodiment.

FIG. 8 is a flowchart of another method, according to an exampleembodiment.

DETAILED DESCRIPTION

Exemplary methods and systems are described herein. It should beunderstood that the word “exemplary” is used herein to mean “serving asan example, instance, or illustration.” Any embodiment or featuredescribed herein as “exemplary” or “illustrative” is not necessarily tobe construed as preferred or advantageous over other embodiments orfeatures. More generally, the embodiments described herein are not meantto be limiting. It will be readily understood that certain aspects ofthe disclosed methods systems and can be arranged and combined in a widevariety of different configurations, all of which are contemplatedherein.

I. OVERVIEW

Illustrative embodiments relate to aerial vehicles, which may be used ina wind energy system, such as an Airborne Wind Turbine (AWT). Inparticular, illustrative embodiments may relate to or take the form ofmethods and systems for transitioning an aerial vehicle between certainflight modes that facilitate conversion of kinetic energy to electricalenergy.

By way of background, an AWT may include an aerial vehicle that flies ina path, such as a substantially circular path, to convert kinetic windenergy to electrical energy. In an illustrative implementation, theaerial vehicle may be connected to a ground station via a tether. Whiletethered, the aerial vehicle can: (i) fly at a range of elevations andsubstantially along the path, and return to the ground, and (ii)transmit electrical energy to the ground station via the tether. (Insome implementations, the ground station may transmit electricity to theaerial vehicle for take-off and/or landing.)

In an AWT, an aerial vehicle may rest in and/or on a ground station (orperch) when the wind is not conducive to power generation. When the windis conducive to power generation, such as when a wind speed may be 3.5meters per second (m/s) at an altitude of 200 meters (m), the groundstation may deploy (or launch) the aerial vehicle. In addition, when theaerial vehicle is deployed and the wind is not conducive to powergeneration, the aerial vehicle may return to the ground station.

Moreover, in an AWT, an aerial vehicle may be configured for hoverflight and crosswind flight. Crosswind flight may be used to travel in amotion, such as a substantially circular motion, and thus may be theprimary technique that is used to generate electrical energy. Hoverflight in turn may be used by the aerial vehicle to prepare and positionitself for crosswind flight. In particular, the aerial vehicle couldascend to a location for crosswind flight based at least in part onhover flight. Further, the aerial vehicle could take-off and/or land viahover flight.

In hover flight, a span of a main wing of the aerial vehicle may beoriented substantially parallel to the ground, and one or morepropellers of the aerial vehicle may cause the aerial vehicle to hoverover the ground. In some implementations, the aerial vehicle mayvertically ascend or descend in hover flight.

In crosswind flight, the aerial vehicle may be propelled by the windsubstantially along a path, which as noted above, may convert kineticwind energy to electrical energy. In some implementations, the one ormore propellers of the aerial vehicle may generate electrical energy byslowing down the incident wind.

The aerial vehicle may enter crosswind flight when (i) the aerialvehicle has attached flow (e.g., steady flow and/or no stall condition(which may refer to no separation of air flow from an airfoil)) and (ii)the tether is under tension. Moreover, the aerial vehicle may entercrosswind flight at a location that is substantially downwind of theground station.

In some implementations, a tension of the tether during crosswind flightmay be greater than a tension of the tether during hover flight. Forinstance, the tension of the tether during crosswind flight may be 15kilonewtons (KN), and the tension of the tether during hover flight maybe 1 KN.

Embodiments described herein relate to transitioning an aerial vehiclefrom hover flight to crosswind flight. In an illustrativeimplementation, a method may involve transitioning an aerial vehiclefrom a hover-flight orientation to a forward-flight orientation andoperating the aerial vehicle in the forward-flight orientation to ascendto a location for crosswind flight. Beneficially, embodiments describedherein may reduce drag on the aerial vehicle during ascent to thelocation for crosswind flight, which may reduce power consumption of theaerial vehicle. Further, embodiments described herein may allow athrust-to-weight ratio of the aerial vehicle to be reduced. Accordingly,the size of one or more components of the aerial vehicle, such as motorsor propellers, may be reduced, which may allow the aerial vehicle to beoptimized for power generation flight.

Moreover, other embodiments may relate to transitioning an aerialvehicle from crosswind flight to hover flight. For instance, someimplementations may involve transitioning an aerial vehicle from acrosswind-flight orientation to a forward-flight orientation andoperating the aerial vehicle in the forward-flight orientation to moveto a location for hover flight. In addition, some implementations mayinvolve transitioning an aerial vehicle from a crosswind-flightorientation to a forward-flight orientation to decelerate the aerialvehicle to a speed for hover flight. Beneficially, such implementationsmay improve entering hover flight by quickly reducing a speed of theaerial vehicle at a low altitude.

II. ILLUSTRATIVE SYSTEMS

A. Airborne Wind Turbine (AWT)

FIG. 1 depicts an AWT 100, according to an example embodiment. Inparticular, the AWT 100 includes a ground station 110, a tether 120, andan aerial vehicle 130. As shown in FIG. 1, the aerial vehicle 130 may beconnected to the tether 120, and the tether 120 may be connected to theground station 110. In this example, the tether 120 may be attached tothe ground station 110 at one location on the ground station 110, andattached to the aerial vehicle 130 at two locations on the aerialvehicle 130. However, in other examples, the tether 120 may be attachedat multiple locations to any part of the ground station 110 and/or theaerial vehicle 130.

The ground station 110 may be used to hold and/or support the aerialvehicle 130 until it is in an operational mode. The ground station 110may also be configured to allow for the repositioning of the aerialvehicle 130 such that deploying of the device is possible. Further, theground station 110 may be further configured to receive the aerialvehicle 130 during a landing. The ground station 110 may be formed ofany material that can suitably keep the aerial vehicle 130 attachedand/or anchored to the ground while in hover flight, forward flight,crosswind flight.

In addition, the ground station 110 may include one or more components(not shown), such as a winch, that may vary a length of the tether 120.For example, when the aerial vehicle 130 is deployed, the one or morecomponents may be configured to pay out and/or reel out the tether 120.In some implementations, the one or more components may be configured topay out and/or reel out the tether 120 to a predetermined length. Asexamples, the predetermined length could be equal to or less than amaximum length of the tether 120. Further, when the aerial vehicle 130lands in the ground station 110, the one or more components may beconfigured to reel in the tether 120.

The tether 120 may transmit electrical energy generated by the aerialvehicle 130 to the ground station 110. In addition, the tether 120 maytransmit electricity to the aerial vehicle 130 in order to power theaerial vehicle 130 for takeoff, landing, hover flight, and/or forwardflight. The tether 120 may be constructed in any form and using anymaterial which may allow for the transmission, delivery, and/orharnessing of electrical energy generated by the aerial vehicle 130and/or transmission of electricity to the aerial vehicle 130. The tether120 may also be configured to withstand one or more forces of the aerialvehicle 130 when the aerial vehicle 130 is in an operational mode. Forexample, the tether 120 may include a core configured to withstand oneor more forces of the aerial vehicle 130 when the aerial vehicle 130 isin hover flight, forward flight, and/or crosswind flight. The core maybe constructed of any high strength fibers. In some examples, the tether120 may have a fixed length and/or a variable length. For instance, inat least one such example, the tether 120 may have a length of 140meters.

The aerial vehicle 130 may be configured to fly substantially along apath 150 to generate electrical energy. The term “substantially along,”as used in this disclosure, refers to exactly along and/or one or moredeviations from exactly along that do not significantly impactgeneration of electrical energy as described herein and/or transitioningan aerial vehicle between certain flight modes as described herein.

The aerial vehicle 130 may include or take the form of various types ofdevices, such as a kite, a helicopter, a wing and/or an airplane, amongother possibilities. The aerial vehicle 130 may be formed of solidstructures of metal, plastic and/or other polymers. The aerial vehicle130 may be formed of any material which allows for a highthrust-to-weight ratio and generation of electrical energy which may beused in utility applications. Additionally, the materials may be chosento allow for a lightning hardened, redundant and/or fault tolerantdesign which may be capable of handling large and/or sudden shifts inwind speed and wind direction. Other materials may be possible as well.

The path 150 may be various different shapes in various differentembodiments. For example, the path 150 may be substantially circular.And in at least one such example, the path 150 may have a radius of upto 265 meters. The term “substantially circular,” as used in thisdisclosure, refers to exactly circular and/or one or more deviationsfrom exactly circular that do not significantly impact generation ofelectrical energy as described herein. Other shapes for the path 150 maybe an oval, such as an ellipse, the shape of a jelly bean, the shape ofthe number of 8, etc.

As shown in FIG. 1, the aerial vehicle 130 may include a main wing 131,a front section 132, rotor connectors 133A-B, rotors 134A-D, a tail boom135, a tail wing 136, and a vertical stabilizer 137. Any of thesecomponents may be shaped in any form which allows for the use ofcomponents of lift to resist gravity and/or move the aerial vehicle 130forward.

The main wing 131 may provide a primary lift for the aerial vehicle 130.The main wing 131 may be one or more rigid or flexible airfoils, and mayinclude various control surfaces, such as winglets, flaps, rudders,elevators, etc. The control surfaces may be used to stabilize the aerialvehicle 130 and/or reduce drag on the aerial vehicle 130 during hoverflight, forward flight, and/or crosswind flight.

The main wing 131 may be any suitable material for the aerial vehicle130 to engage in hover flight, forward flight, and/or crosswind flight.For example, the main wing 131 may include carbon fiber and/or e-glass.Moreover, the main wing 131 may have a variety dimensions. For example,the main wing 131 may have one or more dimensions that correspond with aconventional wind turbine blade. As another example, the main wing 131may have a span of 8 meters, an area of 4 meters squared, and an aspectratio of 15. The front section 132 may include one or more components,such as a nose, to reduce drag on the aerial vehicle 130 during flight.

The rotor connectors 133A-B may connect the rotors 134A-D to the mainwing 131. In some examples, the rotor connectors 133A-B may take theform of or be similar in form to one or more pylons. In this example,the rotor connectors 133A-B are arranged such that the rotors 134A-D arespaced between the main wing 131. In some examples, a vertical spacingbetween corresponding rotors (e.g., rotor 134A and rotor 134B or rotor134C and rotor 134D) may be 0.9 meters.

The rotors 134A-D may configured to drive one or more generators for thepurpose of generating electrical energy. In this example, the rotors134A-D may each include one or more blades, such as three blades. Theone or more rotor blades may rotate via interactions with the wind andwhich could be used to drive the one or more generators. In addition,the rotors 134A-D may also be configured to provide a thrust to theaerial vehicle 130 during flight. With this arrangement, the rotors134A-D may function as one or more propulsion units, such as apropeller. Although the rotors 134A-D are depicted as four rotors inthis example, in other examples the aerial vehicle 130 may include anynumber of rotors, such as less than four rotors or more than fourrotors.

The tail boom 135 may connect the main wing 131 to the tail wing 136.The tail boom 135 may have a variety of dimensions. For example, thetail boom 135 may have a length of 2 meters. Moreover, in someimplementations, the tail boom 135 could take the form of a body and/orfuselage of the aerial vehicle 130. And in such implementations, thetail boom 135 may carry a payload.

The tail wing 136 and/or the vertical stabilizer 137 may be used tostabilize the aerial vehicle and/or reduce drag on the aerial vehicle130 during hover flight, forward flight, and/or crosswind flight. Forexample, the tail wing 136 and/or the vertical stabilizer 137 may beused to maintain a pitch of the aerial vehicle 130 during hover flight,forward flight, and/or crosswind flight. In this example, the verticalstabilizer 137 is attached to the tail boom 135, and the tail wing 136is located on top of the vertical stabilizer 137. The tail wing 136 mayhave a variety of dimensions. For example, the tail wing 136 may have alength of 2 meters. Moreover, in some examples, the tail wing 136 mayhave a surface area of 0.45 meters squared. Further, in some examples,the tail wing 136 may be located 1 meter above a center of mass of theaerial vehicle 130.

While the aerial vehicle 130 has been described above, it should beunderstood that the methods and systems described herein could involveany suitable aerial vehicle that is connected to a tether, such as thetether 120.

B. Illustrative Components of a AWT

FIG. 2 is a simplified block diagram illustrating components of the AWT200. The AWT 200 may take the form of or be similar in form to the AWT100. In particular, the AWT 200 includes a ground station 210, a tether220, and an aerial vehicle 230. The ground station 210 may take the formof or be similar in form to the ground station 110, the tether 220 maytake the form of or be similar in form to the tether 120, and the aerialvehicle 230 may take the form of or be similar in form to the aerialvehicle 130.

As shown in FIG. 2, the ground station 210 may include one or moreprocessors 212, data storage 214, and program instructions 216. Aprocessor 212 may be a general-purpose processor or a special purposeprocessor (e.g., digital signal processors, application specificintegrated circuits, etc.). The one or more processors 212 can beconfigured to execute computer-readable program instructions 216 thatare stored in a data storage 214 and are executable to provide at leastpart of the functionality described herein.

The data storage 214 may include or take the form of one or morecomputer-readable storage media that may be read or accessed by at leastone processor 212. The one or more computer-readable storage media caninclude volatile and/or non-volatile storage components, such asoptical, magnetic, organic or other memory or disc storage, which may beintegrated in whole or in part with at least one of the one or moreprocessors 212. In some embodiments, the data storage 214 may beimplemented using a single physical device (e.g., one optical, magnetic,organic or other memory or disc storage unit), while in otherembodiments, the data storage 214 can be implemented using two or morephysical devices.

As noted, the data storage 214 may include computer-readable programinstructions 216 and perhaps additional data, such as diagnostic data ofthe ground station 210. As such, the data storage 214 may includeprogram instructions to perform or facilitate some or all of thefunctionality described herein.

In a further respect, the ground station 210 may include a communicationsystem 218. The communications system 218 may include one or morewireless interfaces and/or one or more wireline interfaces, which allowthe ground station 210 to communicate via one or more networks. Suchwireless interfaces may provide for communication under one or morewireless communication protocols, such as Bluetooth, WiFi (e.g., an IEEE802.11 protocol), Long-Term Evolution (LTE), WiMAX (e.g., an IEEE 802.16standard), a radio-frequency ID (RFID) protocol, near-fieldcommunication (NFC), and/or other wireless communication protocols. Suchwireline interfaces may include an Ethernet interface, a UniversalSerial Bus (USB) interface, or similar interface to communicate via awire, a twisted pair of wires, a coaxial cable, an optical link, afiber-optic link, or other physical connection to a wireline network.The ground station 210 may communicate with the aerial vehicle 230,other ground stations, and/or other entities (e.g., a command center)via the communication system 218.

In an example embodiment, the ground station 210 may includecommunication systems 218 that allows for both short-range communicationand long-range communication. For example, the ground station 210 may beconfigured for short-range communications using Bluetooth and forlong-range communications under a CDMA protocol. In such an embodiment,the ground station 210 may be configured to function as a “hot spot”; orin other words, as a gateway or proxy between a remote support device(e.g., the tether 220, the aerial vehicle 230, and other groundstations) and one or more data networks, such as cellular network and/orthe Internet. Configured as such, the ground station 210 may facilitatedata communications that the remote support device would otherwise beunable to perform by itself.

For example, the ground station 210 may provide a WiFi connection to theremote device, and serve as a proxy or gateway to a cellular serviceprovider's data network, which the ground station 210 might connect tounder an LTE or a 3G protocol, for instance. The ground station 210could also serve as a proxy or gateway to other ground stations or acommand station, which the remote device might not be able to otherwiseaccess.

Moreover, as shown in FIG. 2, the tether 220 may include transmissioncomponents 222 and a communication link 224. The transmission components222 may be configured to transmit electrical energy from the aerialvehicle 230 to the ground station 210 and/or transmit electrical energyfrom the ground station 210 to the aerial vehicle 230. The transmissioncomponents 222 may take various different forms in various differentembodiments. For example, the transmission components 222 may includeone or more conductors that are configured to transmit electricity. Andin at least one such example, the one or more conductors may includealuminum and/or any other material which allows for the conduction ofelectric current. Moreover, in some implementations, the transmissioncomponents 222 may surround a core of the tether 220 (not shown).

The ground station 210 could communicate with the aerial vehicle 230 viathe communication link 224. The communication link 224 may bebidirectional and may include one or more wired and/or wirelessinterfaces. Also, there could be one or more routers, switches, and/orother devices or networks making up at least a part of the communicationlink 224.

Further, as shown in FIG. 2, the aerial vehicle 230 may include one ormore sensors 232, a power system 234, power generation/conversioncomponents 236, a communication system 238, one or more processors 242,data storage 244, and program instructions 246, and a control system248.

The sensors 232 could include various different sensors in variousdifferent embodiments. For example, the sensors 232 may include a globala global positioning system (GPS) receiver. The GPS receiver may beconfigured to provide data that is typical of well-known GPS systems(which may be referred to as a global navigation satellite system(GNNS)), such as the GPS coordinates of the aerial vehicle 230. Such GPSdata may be utilized by the AWT 200 to provide various functionsdescribed herein.

As another example, the sensors 232 may include one or more windsensors, such as one or more pitot tubes. The one or more wind sensorsmay be configured to detect apparent and/or relative wind. Such winddata may be utilized by the AWT 200 to provide various functionsdescribed herein.

Still as another example, the sensors 232 may include an inertialmeasurement unit (IMU). The IMU may include both an accelerometer and agyroscope, which may be used together to determine the orientation ofthe aerial vehicle 230. In particular, the accelerometer can measure theorientation of the aerial vehicle 230 with respect to earth, while thegyroscope measures the rate of rotation around an axis, such as acenterline of the aerial vehicle 230. IMUs are commercially available inlow-cost, low-power packages. For instance, the IMU may take the form ofor include a miniaturized MicroElectroMechanical System (MEMS) or aNanoElectroMechanical System (NEMS). Other types of IMUs may also beutilized. The IMU may include other sensors, in addition toaccelerometers and gyroscopes, which may help to better determineposition. Two examples of such sensors are magnetometers and pressuresensors. Other examples are also possible.

While an accelerometer and gyroscope may be effective at determining theorientation of the aerial vehicle 230, slight errors in measurement maycompound over time and result in a more significant error. However, anexample aerial vehicle 230 may be able mitigate or reduce such errors byusing a magnetometer to measure direction. One example of a magnetometeris a low-power, digital 3-axis magnetometer, which may be used torealize an orientation independent electronic compass for accurateheading information. However, other types of magnetometers may beutilized as well.

The aerial vehicle 230 may also include a pressure sensor or barometer,which can be used to determine the altitude of the aerial vehicle 230.Alternatively, other sensors, such as sonic altimeters or radaraltimeters, can be used to provide an indication of altitude, which mayhelp to improve the accuracy of and/or prevent drift of the IMU.

As noted, the aerial vehicle 230 may include the power system 234. Thepower system 234 could take various different forms in various differentembodiments. For example, the power system 234 may include one or morebatteries for providing power to the aerial vehicle 230. In someimplementations, the one or more batteries may be rechargeable and eachbattery may be recharged via a wired connection between the battery anda power supply and/or via a wireless charging system, such as aninductive charging system that applies an external time-varying magneticfield to an internal battery and/or charging system that uses energycollected from one or more solar panels.

As another example, the power system 234 may include one or more motorsor engines for providing power to the aerial vehicle 230. In someimplementations, the one or more motors or engines may be powered by afuel, such as a hydrocarbon-based fuel. And in such implementations, thefuel could be stored on the aerial vehicle 230 and delivered to the oneor more motors or engines via one or more fluid conduits, such aspiping. In some implementations, the power system 234 may be implementedin whole or in part on the ground station 210.

As noted, the aerial vehicle 230 may include the powergeneration/conversion components 236. The power generation/conversioncomponents 326 could take various different forms in various differentembodiments. For example, the power generation/conversion components 236may include one or more generators, such as high-speed, direct-drivegenerators. With this arrangement, the one or more generators may bedriven by one or more rotors, such as the rotors 134A-D. And in at leastone such example, the one or more generators may operate at full ratedpower wind speeds of 11.5 meters per second at a capacity factor whichmay exceed 60 percent, and the one or more generators may generateelectrical power from 40 kilowatts to 600 megawatts.

Moreover, as noted, the aerial vehicle 230 may include a communicationsystem 238. The communication system 238 may take the form of or besimilar in form to the communication system 218. The aerial vehicle 230may communicate with the ground station 210, other aerial vehicles,and/or other entities (e.g., a command center) via the communicationsystem 238.

In some implementations, the aerial vehicle 230 may be configured tofunction as a “hot spot”; or in other words, as a gateway or proxybetween a remote support device (e.g., the ground station 210, thetether 220, other aerial vehicles) and one or more data networks, suchas cellular network and/or the Internet. Configured as such, the aerialvehicle 230 may facilitate data communications that the remote supportdevice would otherwise be unable to perform by itself.

For example, the aerial vehicle 230 may provide a WiFi connection to theremote device, and serve as a proxy or gateway to a cellular serviceprovider's data network, which the aerial vehicle 230 might connect tounder an LTE or a 3G protocol, for instance. The aerial vehicle 230could also serve as a proxy or gateway to other aerial vehicles or acommand station, which the remote device might not be able to otherwiseaccess.

As noted, the aerial vehicle 230 may include the one or more processors242, the program instructions 244, and the data storage 246. The one ormore processors 242 can be configured to execute computer-readableprogram instructions 246 that are stored in the data storage 244 and areexecutable to provide at least part of the functionality describedherein. The one or more processors 242 may take the form of or besimilar in form to the one or more processors 212, the data storage 244may take the form of or be similar in form to the data storage 214, andthe program instructions 246 may take the form of or be similar in formto the program instructions 216.

Moreover, as noted, the aerial vehicle 230 may include the controlsystem 248. In some implementations, the control system 248 may beconfigured to perform one or more functions described herein. Thecontrol system 248 may be implemented with mechanical systems and/orwith hardware, firmware, and/or software. As one example, the controlsystem 248 may take the form of program instructions stored on anon-transitory computer readable medium and a processor that executesthe instructions. The control system 248 may be implemented in whole orin part on the aerial vehicle 230 and/or at least one entity remotelylocated from the aerial vehicle 230, such as the ground station 210.Generally, the manner in which the control system 248 is implemented mayvary, depending upon the particular application.

While the aerial vehicle 230 has been described above, it should beunderstood that the methods and systems described herein could involveany suitable aerial vehicle that is connected to a tether, such as thetether 230 and/or the tether 110.

C. Transitioning an Aerial Vehicle from Hover Flight to Crosswind Flight

FIGS. 3 a and 3 b depict an example 300 of transitioning an aerialvehicle from hover flight to crosswind flight, according to an exampleembodiment. Example 300 is generally described by way of example asbeing carried out by the aerial vehicle 130 described above inconnection with FIG. 1. For illustrative purposes, example 300 isdescribed in a series of actions as shown in FIGS. 3 a and 3 b, thoughexample 300 could be carried out in any number of actions and/orcombination of actions.

As shown in FIG. 3 a, the aerial vehicle 130 is connected to the tether120, and the tether 120 is connected to the ground station 110. Theground station 110 is located on ground 302. Moreover, as shown in FIG.3, the tether 120 defines a tether sphere 304 having a radius based on alength of the tether 120, such as a length of the tether 120 when it isextended. Example 300 may be carried out in and/or substantially on aportion 304A of the tether sphere 304. The term “substantially on,” asused in this disclosure, refers to exactly on and/or one or moredeviations from exactly on that do not significantly impacttransitioning an aerial vehicle between certain flight modes asdescribed herein.

Example 300 begins at a point 306 with deploying the aerial vehicle 130from the ground station 110 in a hover-flight orientation. With thisarrangement, the tether 120 may be paid out and/or reeled out. In someimplementations, the aerial vehicle 130 may be deployed when wind speedsincrease above a threshold speed (e.g., 3.5 m/s) at a threshold altitude(e.g., over 200 meters above the ground 302).

Further, at point 306 the aerial vehicle 130 may be operated in thehover-flight orientation. When the aerial vehicle 130 is in thehover-flight orientation, the aerial vehicle 130 may engage in hoverflight. For instance, when the aerial vehicle engages in hover flight,the aerial vehicle 130 may ascend, descend, and/or hover over the ground302. When the aerial vehicle 130 is in the hover-flight orientation, aspan of the main wing 131 of the aerial vehicle 130 may be orientedsubstantially perpendicular to the ground 302. The term “substantiallyperpendicular,” as used in this disclosure, refers to exactlyperpendicular and/or one or more deviations from exactly perpendicularthat do not significantly impact transitioning an aerial vehicle betweencertain flight modes as described herein.

Example 300 continues at a point 308 with while the aerial vehicle 130is in the hover-flight orientation positioning the aerial vehicle 130 ata first location 310 that is substantially on the tether sphere 304. Asshown in FIG. 3 a, the first location 310 may be in the air andsubstantially downwind of the ground station 110.

The term “substantially downwind,” as used in this disclosure, refers toexactly downwind and/or one or more deviations from exactly downwindthat do not significantly impact transitioning an aerial vehicle betweencertain flight modes as described herein.

For example, the first location 310 may be at a first angle from an axisextending from the ground station 110 that is substantially parallel tothe ground 302. In some implementations, the first angle may be 30degrees from the axis. In some situations, the first angle may bereferred to as azimuth, and the first angle may be between 30 degreesclockwise from the axis and 330 degrees clockwise from the axis, such as15 degrees clockwise from the axis or 345 degrees clockwise from theaxis.

As another example, the first location 310 may be at a second angle fromthe axis. In some implementations, the second angle may be 10 degreesfrom the axis. In some situations, the second angle may be referred toas elevation, and the second angle may be between 10 degrees in adirection above the axis and 10 degrees in a direction below the axis.The term “substantially parallel,” as used in this disclosure refers toexactly parallel and/or one or more deviations from exactly parallelthat do not significantly impact transitioning an aerial vehicle betweencertain flight modes described herein.

At point 308, the aerial vehicle 130 may accelerate in the hover-flightorientation. For example, at point 308, the aerial vehicle 130 mayaccelerate up to a few meters per second. In addition, at point 308, thetether 120 may take various different forms in various differentembodiments. For example, as shown in FIG. 3 a, at point 308 the tether120 may be extended. With this arrangement, the tether 120 may be in acatenary configuration. Moreover, at point 306 and point 308, a bottomof the tether 120 may be a predetermined altitude 312 above the ground302. With this arrangement, at point 306 and point 308 the tether 120may not contact the ground 302.

Example 300 continues at point 314 with transitioning the aerial vehicle130 from the hover-flight orientation to a forward-flight orientation,such that the aerial vehicle 130 moves from the tether sphere 304. Asshown in FIG. 3 b, the aerial vehicle 130 may move from the tethersphere 304 to a location toward the ground station 110 (which may bereferred to as being inside the tether sphere 304).

When the aerial vehicle 130 is in the forward-flight orientation, theaerial vehicle 130 may engage in forward flight (which may be referredto as airplane-like flight). For instance, when the aerial vehicle 130engages in forward flight, the aerial vehicle 130 may ascend. Theforward-flight orientation of the aerial vehicle 130 could take the formof an orientation of a fixed-wing aircraft (e.g., an airplane) inhorizontal flight. In some examples, transitioning the aerial vehicle130 from the hover-flight orientation to the forward-flight orientationmay involve a flight maneuver, such as pitching forward. And in such anexample, the flight maneuver may be executed within a time period, suchas less than one second.

At point 314, the aerial vehicle 130 may achieve attached flow. Further,at point 314, a tension of the tether 120 may be reduced. With thisarrangement, a curvature of the tether 120 at point 314 may be greaterthan a curvature of the tether 120 at point 308. As one example, atpoint 314, the tension of the tether 120 may be less than 1 KN, such as500 newtons (N).

Example 300 continues at one or more points 318 with operating theaerial vehicle 130 in the forward-flight orientation to ascend at anangle of ascent AA1 to a second location 320 that is substantially onthe tether sphere 304. As shown in FIG. 3 b, the aerial vehicle 130 mayfly substantially along a path 316 during the ascent at one or morepoints 318. In this example, one or more points 318 is shown as threepoints, a point 318A, a point 318B, and a point 318C. However, in otherexamples, one or more points 318 may include less than three or morethan three points.

In some examples, the angle of ascent AA1 may be an angle between thepath 316 and the ground 302. Further, the path 316 may take variousdifferent forms in various different embodiments. For instance, the path316 may a line segment, such as a chord of the tether sphere 304.

In some implementations, the aerial vehicle 130 may have attached flowduring the ascent. Moreover, in such an implementation, effectiveness ofone or more control surfaces of the aerial vehicle 130 may bemaintained. Further, in such an implementation, example 300 may involveselecting a maximum angle of ascent, such that the aerial vehicle 130has attached flow during the ascent. Moreover, in such animplementation, example 300 may involve adjusting a pitch angle of theaerial vehicle 130 based on the maximum angle of ascent and/or adjustingthrust of the aerial vehicle 130 based on the maximum angle of ascent.In some examples, the adjusting thrust of the aerial vehicle 130 mayinvolve using differential thrusting of one or more of the rotors 134A-Dof the aerial vehicle 130. The pitch angle may be an angle between theaerial vehicle 130 and a vertical axis that is substantiallyperpendicular to the ground 302.

As shown in FIG. 3 b, at point 314 the aerial vehicle 130 may have aspeed V31 and a pitch angle PA31; at point 318A the aerial vehicle 130may have a speed V32 and a pitch angle PA32; at point 318B the aerialvehicle 130 may have a speed V33 and a pitch angle PA33; and at point318C the aerial vehicle 130 may have a speed V34 and a pitch angle PA34.

In some implementations, the angle of ascent AA1 may be selected beforepoint 318A. With this arrangement, the pitch angle PA31 and/or the pitchangle PA32 may be selected based on the angle of ascent AA1. Further, insome examples, the pitch angle PA32, the pitch angle PA33, and/or thepitch angle PA34 may be equal to the pitch angle PA31. However, in otherexamples, the pitch angles PA31, PA32, PA33, and/or PA34 may bedifferent than each other. For instance, the pitch angle PA31 may begreater or less than pitch angles PA32, PA33, and/or PA34; the pitchangle PA32 may be greater or less than pitch angles PA33, PA34, and/orPA31; the pitch angle PA33 may be greater or less than pitch anglesPA34, PA31, and/or PA32; and the pitch angle PA34 may be greater or lessthan pitch angles PA31, PA32, and/or PA33. Further, the pitch angle PA33and/or PA34 may be selected and/or adjusted during the ascent. Furtherstill, the pitch angle PA31 and/or PA32 may be adjusted during theascent.

Moreover, in some implementations, the speed V31 and/or the speed V32may be selected based on the angle of ascent AA1. Further, in someexamples, the speed V32, the speed V33, and the speed V34 may be equalto the speed V31. However, in other examples, speeds V31, V32, V33, andV34 may be different than each other. For example, the speed V34 may begreater than the speed V33, the speed V33 may be greater than the speedV32, and the speed V32 may be greater than the speed V31. Further,speeds V31, V32, V33, and/or V34 may be selected and/or adjusted duringthe ascent.

In some implementations, any or all of the speeds V31, V32, V33, and/orV34 may be a speed that corresponds with a maximum (or full) throttle ofthe aerial vehicle 130. Further, in some implementations, at the speedV32, the aerial vehicle 130 may ascend in a forward-flight orientation.Moreover, at the speed V32, the angle of ascent AA1 may be converged.

As shown in FIG. 3 b, the second location 320 may be in the air andsubstantially downwind of the ground station 110. The second location320 may be oriented with respect to the ground station 110 the similarway as the first location 310 may be oriented with respect to the groundstation 110.

For example, the second location 320 may be at a first angle from anaxis extending from the ground station 110 that is substantiallyparallel to the ground 302. In some implementations, the first angle maybe 30 degrees from the axis. In some situations, the first angle may bereferred to as azimuth, and the angle may be between 30 degreesclockwise from the axis and 330 degrees clockwise from the axis, such as15 degrees clockwise from the axis or 345 degrees clockwise from theaxis.

In addition, as shown in FIG. 3 b, the second location 320 may besubstantially upwind of the first location 310. The term “substantiallyupwind,” as used in this disclosure, refers to exactly upwind and/or oneor more deviations from exactly upwind that do not significantly impacttransitioning an aerial vehicle between certain flight modes asdescribed herein.

At one or more points 318, a tension of the tether 120 may increaseduring the ascent. For example, a tension of the tether 120 at point318C may be greater than a tension of the tether 120 at point 318B, atension of the tether 120 at point 318B may be greater than a tension ofthe tether 120 at point 318A. Further, a tension of the tether 120 atpoint 318A may be greater than a tension of the tether at point 314.

With this arrangement, a curvature of the tether 120 may decrease duringthe ascent. For example, a curvature the tether 120 at point 318C may beless than a curvature the tether at point 318B, and a curvature of thetether 120 at point 318B may be less than a curvature of the tether atpoint 318A. Further, in some examples, a curvature of the tether 120 atpoint 318A may be less than a curvature of the tether 120 at point 314.

Moreover, in some examples, when the aerial vehicle 130 includes a GPSreceiver, operating the aerial vehicle 130 in the forward-flightorientation to ascend at an angle of ascent may involve monitoring theascent of the aerial vehicle 130 with the GPS receiver. With such anarrangement, control of a trajectory of the aerial vehicle 130 duringthe ascent may be improved. As a result, the aerial vehicle 130'sability to follow one or more portions and/or points of the path 316 maybe improved.

Further, in some examples, when the aerial vehicle 130 includes at leastone pitot tube, operating the aerial vehicle 130 in a forward-flightorientation to ascend at an angle of ascent may involve monitoring anangle of attack of the aerial vehicle 130 or a side slip of the aerialvehicle 130 during the ascent with the at least one pitot tube. Withsuch an arrangement, control of the trajectory of the aerial vehicleduring the ascent may be improved. As a result, the aerial vehicle 130'sability to follow one or more portions and/or points of the path 316 maybe improved. The angle of attack may be an angle between a body axis ofthe aerial vehicle 130 and an apparent wind vector. Further, the sideslip may be an angle between a direction substantially perpendicular toa heading of the aerial vehicle 130 and the apparent wind vector.

Example 300 continues at a point 322 with transitioning the aerialvehicle 130 from the forward-flight orientation to a crosswind-flightorientation. In some examples, transitioning the aerial vehicle 130 fromthe forward-flight orientation to the crosswind-flight orientation mayinvolve a flight maneuver.

When the aerial vehicle 130 is in the crosswind-flight orientation, theaerial vehicle 130 may engage in crosswind flight. For instance, whenthe aerial vehicle 130 engages in crosswind flight, the aerial vehicle130 may fly substantially along a path, such as path 150, to generateelectrical energy. In some implementations, a natural roll and/or yaw ofthe aerial vehicle 130 may occur during crosswind flight.

As shown in FIG. 3 b, at points 314-322 a bottom of the tether 120 maybe a predetermined altitude 324 above the ground 302. With thisarrangement, at points 314-322 the tether 120 may not touch the ground302. In some examples, the predetermined altitude 324 may be less thanthe predetermined altitude 312. In some implementations, thepredetermined altitude 324 may be greater than one half of the height ofthe ground station 110. And in at least one such implementation, thepredetermined altitude 324 may be 6 meters.

Thus, example 300 may be carried out so that the tether 120 may notcontact the ground 302. With such an arrangement, the mechanicalintegrity of the tether 120 may be improved. For example, the tether 120might not catch on (or tangle around) objects located on the ground 302.As another example, when the tether sphere 304 is located above a bodyof water (e.g., an ocean, a sea, a lake, a river, and the like), thetether 120 might not be submersed in the water. In addition, with suchan arrangement, safety of one or more people located near the groundstation 110 (e.g., within the portion 304A of the tether sphere 304) maybe improved.

In addition, example 300 may be carried out so that a bottom of thetether 120 remains above the predetermined altitude 324. With such anarrangement, the mechanical integrity of the tether 120 may be improvedas described herein and/or safety of one or more people located near theground station 110 (e.g., within the portion 304A of the tether sphere304) may be improved.

Moreover, one or more actions that correspond with points 306-322 may beperformed at various different time periods in various differentembodiments. For instance, the one or more actions that correspond withpoint 306 may be performed at a first time period, the one or moreactions that correspond with point 308 may be performed at a second timeperiod, the one or more actions that correspond with point 314 may beperformed at a third time period, the one or more actions thatcorrespond with point 318A may be performed at a fourth time period, theone or more actions that correspond with point 318B may be performed ata fifth time period, the one or more actions that correspond with point318C may be performed at a sixth time period, and the one or moreactions that correspond with point 322 may be performed at a seventhtime period. However, in other examples, at least some of the actions ofthe one or more actions that correspond with points 306-322 may beperformed concurrently.

FIGS. 4 a-c are graphical representations involving an angle of ascent,according to an example embodiment. In particular, FIG. 4 a is agraphical representation 402, FIG. 4 b is a graphical representation404, and FIG. 4 c is a graphical representation 406. Each of graphicalrepresentations 402, 404, and 406 may be based on example 300.

More specifically, in FIGS. 4 a-c, an aerial vehicle in an example oftransitioning the aerial vehicle from hover flight to crosswind flightmay have a thrust-to-weight ratio (T/W) of 1.3 and a coefficient of drag(C_(D)) equal to the equation 3+(C_(L) ²/eARn), where C_(L) iscoefficient of lift, e is span efficiency of the aerial vehicle, and ARis aspect ratio of the aerial vehicle. However, in other examples,aerial vehicles described herein may have various other thrust-to-weightratios, such as a thrust-to-weight ratio greater than 1.2. Further, inother examples, aerial vehicles described herein may have various othervalues of C_(D), such as a value of C_(D) between 0.1 and 0.2.

As noted, FIG. 4 a is the graphical representation 402. In particular,the graphical representation 402 depicts an angle of ascent of an aerialvehicle in relation to air speed. In graphical representation 402, theangle of ascent may be measured in degrees, and the airspeed may bemeasured in m/s. As shown in FIG. 4 a, a point 402A on the graphicalrepresentation 402 may represent a maximum angle of ascent of an aerialvehicle for attached flow during an ascent, such as at one or morepoints 318 in example 300. In graphical representation 402, the maximumangle of ascent may be about 65 degrees, and an airspeed thatcorresponds with the maximum angle of ascent may be about 11 m/s.

Moreover, as noted, FIG. 4 b is the graphical representation 404. Inparticular, the graphical representation 404 depicts an angle of ascentof an aerial vehicle in relation to C_(L) of the aerial vehicle. Ingraphical representation 404, the angle of ascent may be measured indegrees, and C_(L) may be a value without a unit of measurement. Asshown in FIG. 4 b, a point 404A on the graphical representation 404 mayrepresent a maximum angle of ascent of an aerial vehicle for attachedflow during an ascent, such as at one or more points 318 in example 300.In graphical representation 404, the maximum angle of ascent may beabout 65 degrees, and the C_(L) that corresponds with the maximum angleof ascent may be about 0.7.

Further, as noted, FIG. 4 c is the graphical representation 406. Inparticular, the graphical representation 406 depicts a first componentof a speed of an aerial vehicle in relation to a second component of thespeed of the aerial vehicle. In graphical representation 406, the firstand second components of speed of the aerial vehicle may be measured inm/s. In some examples, the first component of the speed of the aerialvehicle may be in a direction that is substantially parallel with theground. Further, in some examples, the second component of the speed ofthe aerial vehicle may be in a direction that is substantiallyperpendicular with the ground.

As shown in FIG. 4 c, a point 406A on the graphical representation 406may represent a first and second component of a speed of the aerialvehicle when the aerial vehicle is at a maximum angle of ascent forattached flow during an ascent, such as at one or more points 318 inexample 300. In graphical representation 406, the first component of thespeed of the aerial vehicle that corresponds with the maximum angle ofascent may about 5 m/s, and the second component of the speed of theaerial vehicle that corresponds with the maximum angle of ascent may beabout 10.25 m/s.

FIGS. 5 a and 5 b depict a tether sphere 504, according to an exampleembodiment. In particular, the tether sphere 504 has a radius based on alength of a tether 520, such as a length of the tether 520 when it isextended. As shown in FIGS. 5 a and 5 b, the tether 520 is connected toa ground station 510, and the ground station 510 is located on ground502. Further, as shown in FIGS. 5 a and 5 b, relative wind 503 contactsthe tether sphere 504. In FIGS. 5 a and 5 b, only a portion of thetether sphere 504 that is above the ground 502 is depicted. The portionmay be described as one half of the tether sphere 504.

The ground 502 may take the form of or be similar in form to the ground302, the tether sphere 504 may take the form of or be similar in form tothe tether sphere 304, the ground station 510 may take the form of or besimilar in form to the ground station 110 and/or the ground station 210,and the tether 520 may take the form of or be similar in form to thetether 120 and/or the tether 220.

Examples of transitioning an aerial vehicle between hover flight andcrosswind flight described herein may be carried out in and/orsubstantially on a first portion 504A of the tether sphere 504. As shownin FIGS. 5 a and 5 b, the first portion 504A of the tether sphere 504 issubstantially downwind of the ground station 510. The first portion 504Amay be described as one quarter of the tether sphere 504. The firstportion 504A of the tether sphere 504 may take the form of or be similarin form to the portion 304A of the tether sphere 304.

Moreover, examples of transitioning an aerial vehicle between hoverflight and crosswind flight described herein may be carried out at avariety of locations in and/or on the first portion 504A of the tethersphere 504. For instance, as shown in FIG. 5 a, while the aerial vehicleis in a hover-flight orientation, the aerial vehicle may be positionedat a point 508 that is substantially on the first portion 504A of thetether sphere 504.

Further, as shown in FIG. 5 b, when the aerial vehicle transitions fromthe hover-flight orientation to a forward-flight orientation, the aerialvehicle may be positioned at a point 514 that is inside the firstportion 504A of the tether sphere 504. Further still, as shown in FIG. 5b, when the aerial vehicle ascends in the forward-flight orientation toa point 518 that is substantially on the first portion 504A of thetether sphere 504, the aerial vehicle may follow a path 516. The path516 may take the form of a variety of shapes. For instance, the path 516may be a line segment, such as a chord of the tether sphere 504. Othershapes and/or types of shapes are possible as well.

The point 508 may correspond to point 308 in example 300, the point 514may correspond to point 314 in example 300, the point 518 may correspondto point 318C in example 300, and the path 516 may take the form of orbe similar in form to the path 316.

Further, in accordance with this disclosure, the point 508 and the point518 may be located at various locations that are substantially on thefirst portion 504A of the tether sphere 504, and the point 514 may belocated at various locations that are inside the first portion 504A ofthe tether sphere 504.

D. Transitioning an Aerial Vehicle from Crosswind Flight to Hover Flight

FIGS. 6 a-c depict an example 600 of transitioning an aerial vehiclefrom crosswind flight to hover flight, according to an exampleembodiment. Example 600 is generally described by way of example asbeing carried out by the aerial vehicle 130 described above inconnection with FIG. 1. For illustrative purposes, example 600 isdescribed in a series of actions of the aerial vehicle 130 as shown inFIGS. 6 a-c, though example 600 could be carried out in any number ofactions and/or combination of actions.

As shown in FIG. 6 a, the aerial vehicle 130 is connected to the tether120, and the tether 120 is connected to the ground station 110. Theground station 110 is located on the ground 302. Moreover, as shown inFIG. 6 a, the tether 120 defines the tether sphere 304. Example 600 maybe carried out in and/or substantially on the portion 304A of the tethersphere 304.

Example 600 begins at a point 606 with operating the aerial vehicle 130in a crosswind-flight orientation. When the aerial vehicle is in thecrosswind-flight orientation, the aerial vehicle 130 may engage incrosswind flight. Moreover, at point 606 the tether 120 may be extended.

Example 600 continues at a point 608 with while the aerial vehicle 130is in the crosswind-flight orientation, positioning the aerial vehicle130 at a first location 610 that is substantially on the tether sphere304. (In some examples, the first location 610 may be referred to as athird location). As shown in FIG. 6 a, the first location 610 may in theair and substantially downwind of the ground station 110. The firstlocation 610 may take the form of or be similar in form to the firstlocation 310. However, in some examples, the first location 610 may havean altitude that is greater than an altitude of the first location 310.

For example, the first location 610 may be at a first angle from an axisthat is substantially parallel to the ground 302. In someimplementations, the angle may be 30 degrees from the axis. In somesituations, the first angle may be referred to as azimuth, and the firstangle may be between 30 degrees clockwise from the axis and 330 degreesclockwise from the axis, such as 15 degrees clockwise from the axis or345 degrees clockwise from the axis.

Moreover, at point 606 and point 608, a bottom of the tether 120 may bea predetermined altitude 612 above the ground 302. With thisarrangement, at point 606 and point 608 the tether 120 may not contactthe ground 302. The predetermined altitude 612 may be greater than, lessthan, and/or equal to the predetermined altitude 312.

Example 600 continues at a point 614 with transitioning the aerialvehicle from the crosswind-flight orientation to a forward-flightorientation, such that the aerial vehicle 130 moves from the tethersphere 120. As shown in FIG. 6 b, the aerial vehicle 130 may move fromthe tether sphere 304 to a location toward the ground station 110.

When the aerial vehicle 130 is in the forward-flight orientation, theaerial vehicle may engage in forward flight. In some examples,transitioning the aerial vehicle 130 from the crosswind-flightorientation to the forward-flight orientation may involve a flightmaneuver, such as pitching forward. Further, in such an example, theflight maneuver may be executed within a time period, such as less thanone second.

At point 614, the aerial vehicle 130 may achieve attached flow. Further,at point 314, a tension of the tether 120 may be reduced. With thisarrangement, a curvature of the tether 120 at point 614 may be greaterthan a curvature of the tether 120 at point 608.

Example 600 continues at one or more points 618 with operating theaerial vehicle 130 in the forward-flight orientation to ascend at anangle of ascent AA2 to a second location 620. (In some examples, thesecond location 620 may be referred to as a fourth location). As shownin FIG. 6 b, the aerial vehicle 130 may fly substantially along a path616 during the ascent at one or more points 618. In this example, one ormore points 618 includes two points, a point 618A and point 618B.However, in other examples, one or more points 618 may include less thantwo or more than two points.

In some examples, the angle of ascent AA2 may be an angle between thepath 618 and the ground 302. Further, the path 616 may take variousdifferent forms in various different embodiments. For instance, the path616 may a line segment, such as a chord of the tether sphere 304. Othershapes and/or types of shapes are possible as well. The angle of ascentAA2 may take the form of or be similar in form to the angle of ascentAA1, and the path 616 may take the form of or be similar in form to thepath 316.

In some implementations, at one or more points 618, the aerial vehicle130 may ascend with substantially no thrust provided by the rotors134A-D of the aerial vehicle 130. With this arrangement, the aerialvehicle 130 may decelerate during the ascent. For instance, at one ormore points 618, the rotors 134A-D of the aerial vehicle 130 may beshutoff. The term “substantially no,” as used in this disclosure, refersto exactly no and/or one or more deviations from exactly no that do notsignificantly impact transitioning between certain flight modes asdescribed herein.

Moreover, in some implementations, the aerial vehicle 130 may haveattached flow during the ascent. And in such an implementation,effectiveness of one or more control surfaces of the aerial vehicle 130may be maintained. Further, in such an implementation, example 600 mayinvolve selecting a maximum angle of ascent, such that the aerialvehicle 130 has attached flow during the ascent. Moreover, in such animplementation, example 600 may involve adjusting a pitch angle of theaerial vehicle based on the maximum angle of ascent and/or adjustingthrust of the aerial vehicle 130 based on the maximum angle of ascent.In some examples, the adjusting thrust of the aerial vehicle 130 mayinvolve using differential thrusting of one or more of the rotors 134A-Dof the aerial vehicle 130.

As shown in FIG. 6 b, at point 614 the aerial vehicle 130 may have aspeed V61 and a pitch angle PA61; at point 618A the aerial vehicle 130may have a speed V62 and a pitch angle PA62; and at point 618B theaerial vehicle 130 may have a speed V63 and a pitch angle PA63.

In some implementations, the angle of ascent AA2 may be selected beforepoint 618A. With this arrangement, the pitch angle PA61 and/or the pitchangle PA62 may be selected based on the angle of ascent AA2. Further, insome examples, the pitch angle PA62 and the pitch angle PA63 may beequal to the pitch angle PA61. However, in other examples, the pitchangles PA61, PA62, and PA63 may be different than each other. Forinstance, PA61 may be greater or less than PA62 and/or PA63; PA62 may begreater or less than PA63 and/or PA61; and PA63 may be greater or lessthan PA61 and/or PA62. Further, PA63 may be selected and/or adjustedduring the ascent. Further still, PA61 and/or PA62 may be adjustedduring the ascent.

Moreover, in some implementations, the speed V61 and/or the speed V62may be selected based on the angle of ascent AA2. Further, in someexamples, the speed V62, and the speed V63 may be equal to the speedV61. However, in other examples, the speeds V61, V62, V63 may bedifferent than each other. For example, the speed V63 may be less thanthe speed V62, and the speed V62 may be less than the speed V61.Further, speeds V61, V62, and V63 may be selected and/or adjusted duringthe ascent.

In some implementations, any of speeds V61, V62, and/or V64 may be aspeed that corresponds with a minimum (or no) throttle of the aerialvehicle 130. Further, in some implementations, at the speed V62, theaerial vehicle 130 may ascend in a forward-flight orientation. Moreover,at the speed V62, the angle of ascent AA2 may be converged. As shown inFIG. 6, the second location 620 may be in the air and substantiallydownwind of the ground station 110. The second location 620 may beoriented with respect to the ground station 110 a similar way as thefirst location 610 may be oriented with respect to the ground station110.

For example, the first location 610 may be at a first angle from an axisthat is substantially parallel to the ground 302. In someimplementations, the angle may be 30 degrees from the axis. In somesituations, the first angle may be referred to as azimuth, and the firstangle may be between 30 degrees clockwise from the axis and 330 degreesclockwise from the axis, such as 15 degrees clockwise from the axis or345 degrees clockwise from the axis.

As another example, the first location 610 may be at a second angle fromthe axis. In some implementations, the second angle may be 10 degreesfrom the axis. In some situations, the second angle may be referred toas elevation, and the second angle may be between 10 degrees in adirection above the axis and 10 degrees in a direction below the axis.

At one or more points 618, a tension of the tether 120 may increaseduring the ascent. For example, a tension of the tether 120 at point618B may be greater than a tension of the tether at point 618A, and atension of the tether at point 618A may be greater than a tension of thetether at point 614.

With this arrangement, a curvature of the tether 120 may decrease duringthe ascent. For example, a curvature the tether 120 at point 618B may beless than a curvature of the tether 120 at point 618A. Further, in someexamples, a curvature of the tether 120 at point 618A may be less than acurvature of the tether 120 at point 614.

Moreover, in some examples, when the aerial vehicle 130 includes a GPSreceiver, operating the aerial vehicle 130 in the forward-flightorientation to ascend at an angle of ascent may involve monitoring theascent of the aerial vehicle with the GPS receiver. With such anarrangement, control of a trajectory of the aerial vehicle 130 duringthe ascent may be improved. As a result, the aerial vehicle 130'sability to follow one or more portions and/or portions of the path 616may be improved.

Further, in some examples, when the aerial vehicle 130 includes at leastone pitot tube, operating the aerial vehicle 130 in the forward-flightorientation to ascend at an angle of ascent may involve monitoring anangle of attack of the aerial vehicle 130 or a side slip of the aerialvehicle 130 during the ascent with the at least one pitot tube. Withsuch an arrangement, control of the trajectory of the aerial vehicle 130during the ascent may be improved. As a result, the aerial vehicle'sability to follow one or more portions and/or points of the path 616 maybe improved.

Moreover, as shown in FIG. 6 b, at point 614 and point 618 a bottom ofthe tether 120 may be a predetermined altitude 624 above the ground 302.With this arrangement, at point 614 and point 618 the tether 120 may nottouch the ground 302. In some examples, the predetermined altitude 624may be less than the predetermined altitude 612. And the predeterminedaltitude 624 may be greater than, less than, and/or equal to thepredetermined the predetermined altitude 324. In some implementations,the predetermined altitude 624 may be greater than one half of theheight of the ground station 110. And in at least one suchimplementation, the predetermined altitude 624 may be 6 meters.

Example 600 continues at a point 622 with transitioning the aerialvehicle 130 from the forward-flight orientation to a hover-flightorientation. In some examples, transitioning the aerial vehicle 130 fromthe forward-flight orientation to the hover-flight orientation mayinvolve a flight maneuver. Further, transitioning the aerial vehicle 130from the forward-flight orientation to the hover-flight orientation mayoccur when the aerial vehicle 130 has a threshold speed, such as 15 m/s.In some implementations, transitioning the aerial vehicle 130 from theforward-flight orientation to the hover-flight orientation may occurwhen the speed V63 is 15 m/s. Further, at point 622, a tension of thetether 120 may be greater than a tension of the tether at point 618B.

During the transition from the forward-flight orientation to thehover-flight orientation, the aerial vehicle 130 may be positioned atthird location 624 (In some examples, the third location 624 may bereferred to as a fifth location). As shown in FIG. 6 c, the thirdlocation 624 may be in the air and substantially downwind of the groundstation 110. In some implementations, the third location 624 could bethe same as or similar to the second location 620. When the thirdlocation 624 is not substantially on the tether sphere 304, after point622 the aerial vehicle 130 may be blown by the wind to a fourth location(not shown) that is substantially on the tether sphere 304.

Moreover, as shown in FIG. 6 c, at point 622 a bottom of the tether 120may be a predetermined altitude 626 above the ground 302. With thisarrangement, at point 626 the tether 120 may not touch the ground 302.In some examples, the predetermined altitude 626 may be greater than thepredetermined altitude 612 and/or the predetermined altitude 624.

Thus, example 600 may be carried out so that the tether 120 may notcontact the ground 602. With such an arrangement, the mechanicalintegrity of the tether 120 may be improved. For example, the tether 120might not catch on (or tangle around) objects located on the ground 302.As another example, when the tether sphere 304 is located above a bodyof water described herein, the tether 120 might not be submersed in thewater. In addition, with such an arrangement, safety of one or morepeople located near the ground station 110 (e.g., within the portion304A of the tether sphere 304) may be improved.

In addition, example 600 may be carried out so that a bottom of thetether 120 remains above the predetermined altitude 624. With such anarrangement, the mechanical integrity of the tether 120 may be improvedas described herein and/or safety of one or more people located near theground station may be improved.

Moreover, one or more actions that correspond with points 606-622 may beperformed at various different time periods in various differentembodiments. For instance, the one or more actions that correspond withpoint 606 may be performed at a first time period, the one or moreactions that correspond with point 608 may be performed at a second timeperiod, the one or more actions that correspond with point 614 may beperformed at a third time period, the one or more actions thatcorrespond with point 618A may be performed at a fourth time period, theone or more actions that correspond with point 618B may be performed ata fifth time period, and the one or more actions that correspond withpoint 622 may be performed at a seventh time period. However, in otherexamples, at least some of the actions of the one or more actions thatcorrespond with points 606-622 may be performed concurrently.

Although example 600 has been described above with reference to FIGS. 6a-c, in accordance with this disclosure, point 608 and point 622 mayoccur at various locations that are substantially on the portion 304A ofthe tether sphere 304, and point 614 and one or more points 618 mayoccur at various locations that are inside the portion 304A of thetether sphere.

III. ILLUSTRATIVE METHODS

A. Hover Flight to Crosswind Flight

FIG. 7 is a flowchart illustrating a method 700, according to an exampleembodiment. The method 700 may be used in transitioning an aerialvehicle from hover flight to crosswind flight. Illustrative methods,such as method 700, may be carried out in whole or in part by acomponent or components of an aerial vehicle, such as by the one or morecomponents of the aerial vehicle 130 shown in FIG. 1, the aerial vehicle230 shown in FIG. 2, the ground station 110 shown in FIG. 1, and theground station 210 shown in FIG. 2. For instance, method 700 may beperformed by the control system 248. For simplicity, method 700 may bedescribed generally as being carried out by an aerial vehicle, such asthe aerial vehicle 130 and/or the aerial vehicle 230. However, it shouldbe understood that example methods, such as method 700, may be carriedout by other entities or combinations of entities without departing fromthe scope of the disclosure.

As shown by block 702, method 700 involves operating an aerial vehiclein a hover-flight orientation, wherein the aerial vehicle is connectedto a tether that defines a tether sphere having a radius based on alength of the tether, wherein the tether is connected to a groundstation. At block 702, the aerial vehicle may be operated the same orsimilar way as the aerial vehicle 130 may be operated at point 306 asdescribed with reference to FIG. 3 a.

As shown by block 704, method 700 involves while the aerial vehicle isin the hover-flight orientation, positioning the aerial vehicle at afirst location that is substantially on the tether sphere, wherein thefirst location is substantially downwind of the ground station. At block704, the aerial vehicle may be positioned the same or similar way as theaerial vehicle 130 may be positioned at point 308 as described withreference to FIG. 3 a.

As shown by block 706, method 700 involves transitioning the aerialvehicle from the hover-flight orientation to a forward-flightorientation, such that the aerial vehicle moves from the tether sphere,wherein the aerial vehicle has attached flow, and wherein a tension ofthe tether is reduced. At block 706, the aerial vehicle may betransitioned the same or similar way as the aerial vehicle 130 may betransitioned at point 314 as described with reference to FIG. 3 b.

As shown by block 708, method 700 involves operating the aerial vehiclein the forward-flight orientation to ascend at an angle of ascent to asecond location that is substantially on the tether sphere, wherein thesecond location is substantially downwind of the ground station. Atblock 708, the aerial vehicle may be operated the same or similar way asthe aerial vehicle 130 may be operated at one or more points 318 asdescribed with reference to FIG. 3 b.

For instance, in some embodiments, the aerial vehicle may have attachedflow during the ascent. Moreover, in some embodiments, operating theaerial vehicle in the forward-flight orientation to ascend at the angleof ascent to the second location may involve selecting a maximum angleof ascent, such that the aerial vehicle has attached flow during theascent. And in at least one such embodiment, operating the aerialvehicle in the forward-flight orientation to ascend at the angle ascentto the second location may involve adjusting a pitch angle of the aerialvehicle based on a maximum angle of ascent. Further, in at least onesuch embodiment, operating the aerial vehicle in the forward-flightorientation to ascend at the angle of ascent to the second location mayinvolve adjusting thrust of the aerial vehicle based on the maximumangle of ascent. Further still, in at least one such embodiment,operating the aerial vehicle in the forward-flight orientation to ascendat the angle of ascent to the second location may involve operating theaerial vehicle at a speed that corresponds with a maximum throttle ofthe aerial vehicle.

Further, in some embodiments, the tension of the tether may increaseduring the ascent. Further still, in some embodiments, when the aerialvehicle includes a GPS receiver, operating the aerial vehicle in theforward-flight orientation to ascend at an angle of ascent may involvemonitoring the ascent of the aerial vehicle with the GPS receiver.

Moreover, in some embodiments, when the aerial vehicle includes at leastone pitot tube, operating the aerial vehicle in the forward-flightorientation to ascend at an angle of ascent may involve monitoring anangle of attack of the aerial vehicle or a side slip of the aerialvehicle during the ascent with the at least one pitot tube. Further, insome embodiments, the second location may be substantially upwind of thefirst location.

Further still, in some embodiments, method 700 may be carried out sothat the tether does not contact the ground. Moreover, in someembodiments, method 700 may be carried out so that a bottom of thetether remains above a predetermined altitude.

Moreover, in some embodiments, method 700 may further involvetransitioning the aerial vehicle from the forward-flight orientation toa crosswind-flight orientation. In at least one such embodiment, theaerial vehicle may be transitioned the same or similar way as the aerialvehicle 130 may be transitioned at point 322 as described with referenceto FIG. 3 b.

And in some embodiments, method 700 may further involve deploying theaerial vehicle from a ground station in the hover-flight orientation. Inat least one such embodiment, the aerial vehicle may be deployed thesame or similar way as the aerial vehicle 130 may be deployed at point306 as described with reference to FIG. 3 a.

B. Crosswind Flight to Hover Flight

FIG. 8 is a flowchart illustrating a method 800, according to an exampleembodiment. The method 800 may be used in transitioning an aerialvehicle from crosswind flight to hover flight. Illustrative methods,such as method 800, may be carried out in whole or in part by acomponent or components of an aerial vehicle, such as by the one or morecomponents of the aerial vehicle 130 shown in FIG. 1, the aerial vehicle230 shown in FIG. 2, the ground station 110 shown in FIG. 1, and theground station 210 shown in FIG. 2. For instance, method 800 may beperformed by the control system 248. For simplicity, method 800 may bedescribed generally as being carried out by an aerial vehicle, such asthe aerial vehicle 130 and/or the aerial vehicle 230. However, it shouldbe understood that example methods, such as method 800, may be carriedout by other entities or combinations of entities without departing fromthe scope of the disclosure.

As shown by block 802, method 800 involves operating an aerial vehiclein a crosswind-flight orientation, wherein the aerial vehicle isconnected to a tether that defines a tether sphere having a radius basedon a length of the tether, wherein the tether is connected to a groundstation. At block 802, the aerial vehicle may be operated the same orsimilar way as the aerial vehicle 130 may be operated at point 606 asdescribed with reference to FIG. 6 a.

As shown by block 804, method 800 involves while the aerial vehicle isin the crosswind-flight orientation, positioning the aerial vehicle at afirst location that is substantially on the tether sphere, wherein thefirst location is substantially downwind of the ground station. At block804, the aerial vehicle may be positioned the same or similar way as theaerial vehicle 130 may be positioned at point 608 as described withreference to FIG. 6 a.

As shown by block 806, method 800 involves transitioning the aerialvehicle from the crosswind-flight orientation to a forward-flightorientation, such that the aerial vehicle moves from the tether sphere,wherein the aerial vehicle has attached flow, and wherein a tension ofthe tether is reduced. At block 806, the aerial vehicle may betransitioned the same or similar way as the aerial vehicle 130 may betransitioned at point 614 as described with reference to FIG. 6 b.

As shown by block 808, method 800 involves operating the aerial vehiclein the forward-flight orientation to ascend at an angle of ascent to asecond location, wherein the second location is substantially downwindof the ground station. At block 808, the aerial vehicle may be operatedthe same or similar way as the aerial vehicle may be operated at one ormore points 618 as described with reference to FIG. 6 b.

For instance, in some embodiments, the aerial vehicle may have attachedflow during the ascent. Moreover, in some embodiments, operating theaerial vehicle in the forward-flight orientation to ascend at the angleof ascent to the second location may involve selecting a maximum angleof ascent, such that the aerial vehicle has attached flow during theascent. And in at least one such embodiment, operating the aerialvehicle in the forward-flight orientation to ascend at the angle ascentto the second location may involve adjusting a pitch angle of the aerialvehicle based on the maximum angle of ascent. Further, in at least onesuch embodiment, operating the aerial vehicle in the forward-flightorientation to ascend at the angle of ascent to the second location mayinvolve adjusting thrust of the aerial vehicle based on the maximumangle of ascent. Further still, in at least one such embodiment,operating the aerial vehicle in the forward-flight orientation to ascendat the angle of ascent to the second location may involve operating theaerial vehicle with substantially no thrust.

Further, in some embodiments, the tension of the tether may increaseduring the ascent. Further still, in some embodiments, when the aerialvehicle includes a GPS receiver, operating the aerial vehicle in theforward-flight orientation to ascend at an angle of ascent may involvemonitoring the ascent of the aerial vehicle with the GPS receiver.

Moreover, in some embodiments, when the aerial vehicle includes at leastone pitot tube, operating the aerial vehicle in the forward-flightorientation to ascend at an angle of ascent may involve monitoring anangle of attack of the aerial vehicle or a side slip of the aerialvehicle during the ascent with the at least one pitot tube. Further, insome embodiments, the second location may be substantially upwind of thefirst location.

Further still, in some embodiments, method 800 may be carried out sothat the tether does not contact the ground. Moreover, in someembodiments, method 800 may be carried out so that a bottom of thetether remains above a predetermined altitude.

And in some embodiments, method 800 may further involve transitioningthe aerial vehicle from the forward-flight orientation to a hover-flightorientation. In at least one such embodiment, the aerial vehicle may betransitioned the same or similar way as the aerial vehicle 130 may betransitioned at point 622 as described with reference to FIG. 6 c.

VI. CONCLUSION

The particular arrangements shown in the Figures should not be viewed aslimiting. It should be understood that other embodiments may includemore or less of each element shown in a given Figure. Further, some ofthe illustrated elements may be combined or omitted. Yet further, anexemplary embodiment may include elements that are not illustrated inthe Figures.

Additionally, while various aspects and embodiments have been disclosedherein, other aspects and embodiments will be apparent to those skilledin the art. The various aspects and embodiments disclosed herein are forpurposes of illustration and are not intended to be limiting, with thetrue scope and spirit being indicated by the following claims. Otherembodiments may be utilized, and other changes may be made, withoutdeparting from the spirit or scope of the subject matter presentedherein. It will be readily understood that the aspects of the presentdisclosure, as generally described herein, and illustrated in thefigures, can be arranged, substituted, combined, separated, and designedin a wide variety of different configurations, all of which arecontemplated herein.

1. A method comprising: operating an aerial vehicle in a hover-flightorientation, wherein the aerial vehicle is connected to a tether thatdefines a tether sphere having a radius based on a length of the tether,wherein the tether is connected to a ground station; while the aerialvehicle is in the hover-flight orientation, positioning the aerialvehicle at a first location that is substantially on the tether sphere,wherein the first location is substantially downwind of the groundstation; transitioning the aerial vehicle from the hover-flightorientation to a forward-flight orientation, such that the aerialvehicle moves from the tether sphere, wherein the aerial vehicle hasattached flow, and wherein a tension of the tether is reduced; andoperating the aerial vehicle in the forward-flight orientation to ascendat an angle of ascent to a second location that is substantially on thetether sphere, wherein the second location is substantially downwind ofthe ground station.
 2. The method of claim 1, wherein the aerial vehiclehas attached flow during the ascent.
 3. The method of claim 1, whereinoperating the aerial vehicle in the forward-flight orientation to ascendat the angle of ascent to the second location comprises selecting amaximum angle of ascent, such that the aerial vehicle has attached flowduring the ascent.
 4. The method of claim 3, wherein operating theaerial vehicle in the forward-flight orientation to ascend at the angleof ascent to the second location comprises adjusting a pitch angle ofthe aerial vehicle based on the maximum angle of ascent.
 5. The methodof claim 3, wherein operating the aerial vehicle in the forward-flightorientation to ascend at the angle of ascent to the second locationcomprises adjusting thrust of the aerial vehicle based on the maximumangle of ascent.
 6. The method of claim 1, wherein the tension of thetether increases during the ascent.
 7. The method of claim 1, whereinthe aerial vehicle comprises a global positioning system (GPS) receiver,and wherein operating the aerial vehicle in the forward-flightorientation to ascend at an angle of ascent comprises monitoring theascent of the aerial vehicle with the GPS receiver.
 8. The method ofclaim 1, wherein the aerial vehicle comprises at least one pitot tube,and wherein operating the aerial vehicle in the forward-flightorientation to ascend at an angle of ascent comprises monitoring anangle of attack of the aerial vehicle or a slide slip of the aerialvehicle during the ascent with the at least one pitot tube.
 9. Themethod of claim 1, wherein the second location is substantially upwindof the first location.
 10. The method of claim 1, carried out whereinthe tether does not contact the ground.
 11. The method of claim 1,carried out wherein a bottom of the tether remains above a predeterminedaltitude.
 12. The method of claim 1, further comprising transitioningthe aerial vehicle from the forward-flight orientation to acrosswind-flight orientation.
 13. A system comprising: a tetherconnected to a ground station, wherein the tether defines a tethersphere having a radius based on a length of the tether; an aerialvehicle connected to the tether; and a control system configured to:operate the aerial vehicle in a hover-flight orientation; while theaerial vehicle is in the hover-flight orientation, position the aerialvehicle at a first location that is substantially on the tether sphere,wherein the first location is substantially downwind of the groundstation; transition the aerial vehicle from the hover-flight orientationto a forward-flight orientation, such that the aerial vehicle moves fromthe tether sphere, wherein the aerial vehicle has attached flow, andwherein a tension of the tether is reduced; and operate the aerialvehicle in the forward-flight orientation to ascend at an angle ofascent to a second location that is substantially on the tether sphere,wherein the second location is substantially downwind of the groundstation.
 14. The system of claim 13, wherein operating the aerialvehicle in the forward-flight orientation to ascend at the angle ofascent to the second location comprises selecting a maximum angle ofascent, such that the aerial vehicle has attached flow during theascent.
 15. The system of claim 13, wherein operating the aerial vehiclein the forward-flight orientation to ascend at the angle of ascent tothe second location comprises adjusting a pitch angle of the aerialvehicle based on the maximum angle of ascent.
 16. The system of claim13, wherein operating the aerial vehicle in the forward-flightorientation to ascend at the angle of ascent to the second locationcomprises adjusting thrust of the aerial vehicle based on the maximumangle of ascent.
 17. The system of claim 13, carried out wherein thetether does not contact the ground.
 18. The system of claim 13, carriedout wherein a bottom of the tether remains above a predeterminedaltitude.
 19. The system of claim 13, wherein the control system isfurther configured to transition the aerial vehicle from theforward-flight orientation to a crosswind-flight orientation.
 20. Amethod comprising: operating an aerial vehicle in a crosswind-flightorientation, wherein the aerial vehicle is connected to a tether thatdefines a tether sphere having a radius based on a length of the tether,wherein the tether is connected to a ground station; while the aerialvehicle is in the crosswind-flight orientation, positioning the aerialvehicle at a first location that is substantially on the tether sphere,wherein the first location is substantially downwind of the groundstation; transitioning the aerial vehicle from the crosswind-flightorientation to a forward-flight orientation, such that the aerialvehicle moves from the tether sphere, wherein the aerial vehicle hasattached flow, and wherein a tension of the tether is reduced; andoperating the aerial vehicle in the forward-flight orientation to ascendat an angle of ascent to a second location, wherein the second locationis substantially downwind of the ground station.